1. Field of the Invention
The structure of this invention resides in the art of fabric-covered corrugated drainage structures and more particularly relates to corrugated pipes and other corrugated products which are covered with a fabric material with a separation grid structure provided between the peak portions of the corrugations and the fabric covering to promote fluid flow from inside the covered structure to all areas of its fabric covering.
2. Description of the Prior Art
Corregated pipes have wide use in disposal and irrigation systems. In disposal systems perforated corrugated pipes are used and are commonly made of a thermoplastic material such as ABS, styrene polymers, polyethelene or equivalent some of which materials have carbon mixed therewith. In septic systems a house sewer line extends to a septic tank which provides for the separation therein of sewerage into three layers: a solid sludge which settles therein, an upper floating scum layer, and a middle layer of effluent which is carried out of the septic tank through an outlet pipe to a drainage field system. In the drainage field the effluent is carried through a series of perforated pipes which are buried within drainage trenches in the ground. The effluent passes through the perforations in the pipes and seeps into the earth where impurities in such fluid are neutralized. Pipes used in such drainage field systems are frequently of corrugated construction since such pipe construction provides great strength allowing for a thinner pipe wall than would be required for a non-corrugated perforated pipe to support the load that occurs on the ground thereabove. Over time the pipe's perforations can become plugged, and it is expected that at some time within a twenty year period a typical drainage field will have to be dug up and its pipes replaced due to blockage of the perforations in the perforated pipes. In some situations a biological mat forms at the interface of the liquid and the soil.
To keep dirt and gravel out of the valleys of the corrugations of the corrugated pipe and to assist in the formation of the biological mat, many corrugated pipes are wrapped with a woven or non-woven synthetic porous fabric material. Such material can be a porous netting where the pores are sufficiently fine so as to prevent soil and/or gravel from entering the valley areas of the corrugated pipe but yet large enough for fluid to pass therethrough. Such wrapped corrugated pipes are well known in the industry such as those manufactured by Hancor, Inc. of Findlay, Ohio. As the effluent passes through the perforations in the corrugated pipe and then through the fabric material, the biological mat forms on the fabric material which mat helps to react with, and neutralize, the effluent.
A cross-section along the axis of a typical piece of corrugated pipe shows spaced apart walls that extend from adjacent peaks down to a valley portion therebetween. In one embodiment, the walls generally converge symmetrically from adjacent peaks to the valley therebetween at about a 3 degree angle. Each peak can have a top portion with a generally undulating cross-section between adjacent valley sidewall portions. The peak's top portion has a first generally rounded, outwardly extending rib section adjacent to each sidewall which sidewalls extend down forming each valley and between each of these rib sections formed on each side of each peak is an inwardly extending rounded recess. The ribs and recess formed across the top of the peak help provide increased strength to the corrugated structure. The valley portions which are at the base of each of the sidewalls between adjacent peak areas are generally flat or slightly rounded in cross-section and have a series of perforations therein for fluid passage. The corrugations can have a sidewall height equivalent to the distance between the outer surfaces of top ribs facing each other on adjacent peaks, and the opening at the top of each valley can generally be of the same width as the width of the top portion of each peak. It should be noted that other cross-sectional formulas for the construction of the corrugated pipe can be utilized with such formulas relating the valley depth to the distance between corrugated peaks. It should be noted that a pipe structure with valleys and wide peaks, each peak with a recess therein, has substantially more strength than a pipe constructed of the same thickness of material but which is cylindrical with flat walls in cross-section. It is the desirable features of corrugated pipes, namely their light weight, cost-effectiveness and ease of handling and transportation that have made them desirable for use in drainage field systems and irrigation systems.
As mentioned above, in many instances the corrugated pipes are covered with a porous woven or non-woven synthetic fabric material to help keep soil and gravel out of the valleys of the corrugations and further to help form the biological mat for the breakdown of the waste products in the effluent from a septic tank. The perforations that are located in the bottoms of the valleys of corrugated pipe allow the fluid to pass out of the pipe to the area formed by the bottom of the valley and the side walls of the corrugation up to the sides of the peaks of the corrugations where the fluid contacts the fabric material and then passes therethrough. The fluid can only pass through the areas of the fabric where the fluid has direct contact with the fabric which is only in the area specifically above the valleys. Since no fluid passes beyond the rib areas on each side of a valley, no fluid passes through the porous fabric material that is placed over the width of the peaks of the corrugations. Thus the effluent can pass out of only approximately 50% of the exterior wrapped surface of the pipe using a one-to-one valley top width to peak width ratio. The fluid passing from the valley areas exits only through the fabric directly above the valleys which form the only areas that a biological mat can be created with no biological mat being formed in the areas of the fabric extending over the peaks which area is approximately, in the example used, 50% of the wrapped corrugated pipe's exterior surface. Therefore a problem that arises in the prior art is that usually only 50% of the exterior surface of the wrapped corrugated pipe is available for fluid drainage and for the formation of the biological mat thereupon.